GAZA, July 26 (Reuters) - Israeli tank shelling killed 18 Palestinians from a single family in the southern Gaza Strip on Saturday, the Gaza health ministry said, shortly before a 12-hour humanitarian truce agreed by Israel and Hamas went into effect.

Gaza Health Ministry spokesman Ashraf Al-Qidra said that the 18 members of the Al-Najar family had been trapped inside their house in Khuzaa village east of Khan Younis since Thursday and that many others were wounded in Israeli tank shelling.

The fatalities brought up to 881 the number of Palestinians, most of them civilians, killed since the Israeli Gaza offensive began on July 8. (Reporting by Nidal Almughrabi; writing by Maayan Lubell; Editing by Ron Popeski)